1. Field of the Invention
The present invention relates generally to the detection of small amounts of components in a gaseous mixture. More particularly, it relates to an improved system, detector and detecting method for gas chromatography applications. Most especially, it relates to such an improved system, detector and detecting method for the detection of halogen containing substances.
2. Description of the Prior Art
The art relating to detectors and detecting methods for substances in mixtures is a well-developed one. The following issued patents and other publications give an indication of the scope and content of this prior art.
U.S. Pat. No. 1,421,720, issued July 4, 1920 to C. H. M. Roberts, discloses a detector having two electrodes with an air gap through which a current flows. The magnitude of the current is dependent on the molecular weight of a gas in the gap. The current source in this detector is emitted electrons from a heated, negatively charged electrode. Apparently, temperatures tested with this device were insufficiently high to create selective ionization of halogens. Current is increased by increasing the temperature of the emitting electrode and by increasing the potential gradient across the gap between the electrodes. The presence of chlorine in helium causes a decrease in the emission current of this detector.
U.S. Pat. No. 1,809,115, issued June 9, 1931 to R. H. Goddard, discloses a device for the generation of positive ions using an evaporation source for deposition on a thin platinum cylinder. Volatile metallic substances penetrate the platinum. There is no mention of halogens.
U.S. Pat. No. 2,334,356, issued Nov. 28, 1941 to B. Salzberg et al., discloses an electron emitting vacuum gauge with multiple grids.
U.S. Pat. No. 2,550,498, issued Apr. 24, 1951 to C. W. Rice, discloses a halogen detector based on the generation of positive ions at a heated anode. The presence of an activating material containing alkali metal ions, such as sodium, is needed to produce the positive ions. The activating material is in the form of an impregnated ceramic. A temperature as high as 1200 degrees K. for the anode is required. The electrodes are in the form of filaments, both of which may be heated.
U.S. Pat. No. 2,597,352, issued Dec. 18, 1951 to W. C. White, discloses a detector for trace substances in an atmosphere using intermittent exposure of the detector to the contaminant to avoid variations in response due to excessive depletion of the sensitizer on the anode surface. This disclosure gives a good understanding of the process as developed at that time. U.S. Pat. No. 2,591,485, issued Apr. 26, 1952 to W. C. White, discloses a vacuum leak detector sensitive to halogens, with a slightly different configuration than previous devices.
U.S. Pat. No. 2,795,716, issued June 11, 1957 to J. A. Roberts, discloses an electrical vapor detector similar to that described in the above Rice patent. Detail concerning sensitizing substances for the detection of halogens is given. The temperature range of 700 to 925 degrees C. for the emitter is disclosed. A high alumina content for the core is recommended. The lithium containing metal glass known as b-eucryptite is mentioned. Sensitivity to the four common halogens is demonstrated. This patent emphasizes the composition and configuration of the detector.
U.S. Pat. No. 2,814,018, issued Nov. 19, 1957 to P. D. Zemany, discloses a detector similar to that of the Rice patent, but operated at a reduced pressure, that gives an electron discharge for the detection of halogens. Only the anode is heated.
U.S. Pat. No. 2,897,437, issued July 28, 1959 to W. E. Briggs et al., discloses a system for vacuum leak detection based on the concept of the Rice patent. By keeping signals low, depletion of the sensitizing material on the anode is avoided, resulting in better reproducibility.
U.S. Pat. No. 2,928,042, issued Mar. 8, 1960 to R. B. Lawrence et al., discloses a high vacuum device for the detection of halogens in the form of a leak detector. A heated platinum anode is used.
U.S. Pat. Nos. 3,009,074, issued Nov. 14, 1961; 3,065,411, issued Nov. 20, 1962; 3,071,722, issued Jan. 1, 1963 and 3,144,600, issued Aug. 11, 1964, all to J. A. Roberts, disclose the use of Nichrome V wire electrodes and magnesium silicate ceramics in a simple to manufacture halogen electrical vapor detector, an indicating circuit for a leak detector, additional details for the construction of a practical leak detector, and an electronic circuit to adjust for background and range to give more reproducible and linear response, respectively.
A. Karmen and L. Giuffrida, Nature, 201, 1204 (1964) disclose the enhancement of the response of a hydrogen flame ionization detector to compounds containing halogens and phosphorus. Sodium vapor is introduced from a heated probe. This is probably the first thermionic detector for gas chromatography. A. Karmen, Anal. Chem. 36, 1416 (1964) discloses a specific detector for halogens and phosphorus with two flame detectors in tandem. U.S. Pat. No. 3,372,994, issued Mar. 12, 1968 to L. Guiffrida discloses a modified flame ionization detector with a heated electrode coated with a fused alkali metal salt.
C. H. Hartman, J. Chrom. Sci. 7, 163 (1969), describes an alkali flame detector with a salt tip on the flame input capillary. CsBr is used for detecting phosphorus, and Rb.sub.2 SO.sub.4 is used for detecting nitrogen
B. Kolb and J. Bischoff, J. Chrom. Sci. 12, 625 (1974), disclose a new design of a thermionic nitrogen and phosphorus detector for gas chromatography. They concluded that the mechanism involves gaseous alkali atoms followed by transfer of an electron to a more electronegative gaseous molecule. The emitter electrode is maintained at -130 V. with respect to the collector electrode The emitter electrode is an electrically heated glass bead, containing Rb silicate. B. Kolb, M. Auer and P. Posposil, J. Chrom. Sci. 15, 53 (1977) disclose a tunable detector selective for carbon, nitrogen and phosphorus. An alkali glass bead source is used. A list of electron affinities is given. B. Kolb, M. Auer and P. Posposil, J. Chromatography 134, 65 (1977) describe a detector that can be operated as either a flame ionization detector or a thermionic detector. They use a Rb bead heated on a coil and negatively charged. K. Olah, A. Skoze and Zs. Vajta, J. Chrom. Sci. 17, 497 (1979), describe the ionization mechanism of the Kolb device. They assume a cyclic process or reduction of alkali ions at the emitter cathode.
C. A. Burgett, D. H. Smith and H. P. Bente, J. Chromatography 134, 57 (1977), describe a new nitrogen and phosphorus detector and its applications to gas chromatography. The collector is polarized at -240 V., and it is assumed that M+ ions are formed from alkali atoms and recollected on the negatively charged collector cylinder.
P. L. Patterson, J. Chromatography 167, 381 (1978), discloses the selective responses of a flameless thermionic detector. He reports that the emitter gives too high backgrounds when charged positively. A negatively charged emitter is used. P. L. Patterson and R. L. Howe, J. Chrom. Sci. 16, 275 (1978), describe a thermionic nitrogen and phosphorus detector with an alkali-ceramic bead. The voltage applied to the emitter is 0 to -12 V. This is a form of hydrogen flame ionization detector. More positive voltages increase the background current to levels that "mask sample response". U.S. Pat. No. 4,203,726, issued May 3, 1980 to P. L. Patterson, discloses a detector configured like a hydrogen flame ionization detector and having a negatively charged bead emitter that is heated with a resistance wire that also serves as the cathode. The background current increases with temperature according to the Richardson-Dushman equation, i.e., a plot of log (current) against 1/T is a linear function (e.sup.-W/T), where W is the work function P. L. Patterson et al., J. Chrom. Sci. 20, 97 (1982), describe a thermionic ionization detector with a cylindrical rod thermionic source rather than a bead, used with a low flow of hydrogen (3-6 ml/min.) and an air flow of 150-200 ml/min. A temperature of 400-600 degrees C. in one mode of operation and 600-800 degrees C. in another mode. Negative ions are reported to be formed from electronegative substances. This detector is sensitive to nitrogen, phosphorus and halogens and uses a cesium activator. U.S. Pat. No. 4,524,047, issued June 18, 1985 to P. L. Patterson, discloses a hydrogen flame ionization like thermionic detector with a multiple layered ionization source and a ceramic rod heated internally with the cathode. A temperature range of 100 to 1000 degrees C. is disclosed. The ceramic rod contains an alkali metal compound in primarily alumina. Electric currents originate on the surface of the ceramic coating P. L. Patterson, J. Chrom. Sci. 24, 41 (1986), reviews recent advances in thermionic detection. Four configurations suitable for mounting on a classical hydrogen flame ionization detector are described. In every case, the emitter electrode is also used to provide heat with a heating current flowing through the negatively charged emitter. P. L. Patterson, J. Chrom. Sci. 24, 466 (1986), reviews the common detectors, including photo-ionization detectors, flame ionization detectors, electron-capture detectors and thermionic ionization detectors. The surface temperature of the emitter of the thermionic ionization detector is from 400 to 800 degrees C., and the emitter is maintained negatively to avoid high background currents. He teaches that, "All modern TIDs employ a solid surface composed of a ceramic or glass matrix molded onto an electrical heater wire." He also teaches that, "The ionization produced in the TID is the result of a gas-solid interaction, so there is no indication that equal concentrations of positive and negative ion species are formed, as in the case of the FID and PID." The latter statement is incorrect, since a balance of charges is always required. This demonstrates that TID is still poorly understood.
T. Fujii and H. Arimoto, Anal. Chem. 57, 490 (1985), describe a thermionic detector based on a lanthanum boride bead. They state that their results are consistent with a process of negative surface ionization. T. Fujii and H. Arimoto, Anal. Chem. 57, 2625 (1985), describe a surface ionization detector with a hot platinum, positively charged emitter. They indicate that organic molecules are ionized on the hot metal surface T. Fujii and H. Arimoto, J. Chromatography 355, 375 (1986), found that Ir is a better emitter than Pt in a surface ionization detector. Oxygen improves the surface ionization, and temperatures of 650 degrees C. were used, giving the highest detector response.
While a substantial amount of work has been done with prior art systems, detectors and detecting methods, detector configurations and detecting methods at present do not incorporate optimum conditions for detecting many compounds, for example, organic compounds containing halogens.